At present, for an anesthesia machine or a ventilator, there exists two methods for detecting tidal volume:
A sensor is placed at the patient terminal for real-time detecting data, which brings about the advantages that the monitored data is accurate, data process is not necessary, and the patient's status can be reflected in real-time. However, this method causes the disadvantage that the circuit from the machine to the patient's mouth is relatively long, resulting in possible interference during an operation by a surgeon. Furthermore, the relatively long circuit may affect the precision of the signals.
A sensor is embedded into the breathing circuit for real-time detecting data, which brings about the advantages that the problem of interference is eliminated and the signal is stable while there is disadvantage that the monitored data is not the real data of the patient, thus requiring a calculation to process it.
Generally, the compliance C is used for the compensation calculation of tidal volume.
The system compliance C=ΔV/(Pplate−PEEP).
wherein ΔV is the tidal volume variation, Pplate is the plateau pressure, and PEEP is the positive end expiratory pressure. ΔV, Pplate and PEEP can be obtained through the monitoring of the sensor.VT=ΔV×(C−Ctube)/C 
wherein VT is the calculated tidal volume of the patient, Ctube is the compliance of the circuit and which is obtained by self-checking when the machine starts up.
The disadvantage of such calculation is that the calculated tidal volume VT of patient is not accurate.